Typically, electric vehicles drive electric motors based on electric power stored in batteries having a limited power storage capability.
Typically, vehicles with traction control systems control drive torque delivered by the vehicle drive wheels to limit the drive torque to an amount at which the vehicle drive wheels have traction on the driving surface. Or, in other words, the amount of available drive torque is limited by the coefficient of a friction between the drive wheels and the road. Thus, traction control systems have a motive force source such as a vehicle engine and seek, during traction control, to limit the output torque of the motive force source to a force that would drive the wheels without slip on the road surface. Some traction control systems include brake control which allows faster reduction of torque delivered by the drive wheels while positive torque is maintained from the motive force. The wheels must have positive torque to provide motive force and motor/transmission/brake controls are used to limit the amount of that torque.
In vehicles with traction control, it is known that it is desirable to detect the presence of a spare tire on the vehicle because the spare tire may be undersized and cause a wheel speed differential between that tire and other tires on the vehicle due to the different tire sizes. This wheel speed differential, if not accounted for in the traction control system may cause the vehicle to unnecessarily enter traction control mode, which will cause an undesirable loss of drive torque for the vehicle.
For example, the vehicle may accelerate normally until the vehicle hits a speed at which the wheel speed differential between the spare and the remaining tires causes traction control to turn on, at which time, drive torque will be reduced from the motor until the wheel speed differential is low enough to cause traction control to turn off. Thus, for example, the vehicle may speed up to a speed of 40-45 m.p.h. and then slow down due to motor torque reduction and/or traction control braking until the vehicle speed is 10-15 m.p.h., at which case, traction control will turn off and the vehicle will be allowed to speed up again until traction control turns on.
A known method for compensating for the spare tire is to determine the presence of a spare tire by monitoring the wheel speeds and determining if one wheel, in a normal operating condition, i.e., non-traction control and non-ABS, is spinning faster than the remaining wheels. If so, then it is determined that this wheel is an undersized spare tire and its wheel speed is normalized to match the other wheel speed. That is, a proportional constant between that wheel speed and the other wheel speeds is determined and multiplied by that wheel speed to normalize the resultant signal. The normalized signal is then used with the remaining signals to determine whether or not traction control should be activated and to determine the traction control command.